Multi-piece solid golf ball

ABSTRACT

The golf ball includes a core, an intermediate layer, and a cover, in which the core is formed of a rubber composition into a single layer or a plurality of layers, the intermediate layer and the cover are both formed of a single-layer resin composition, and a relationship between an initial velocity of the core, an initial velocity of a sphere (intermediate layer-encased sphere) in which the core is encased with the intermediate layer, and an initial velocity of a sphere (ball) in which the intermediate layer-encased sphere is encased with the cover satisfies the following two conditions: 
       (initial velocity of ball)&lt;(initial velocity of intermediate layer-encased sphere) 
       0.60≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).
         and the specific deflection is 3.00 mm or less, and a specific gravity of the intermediate layer is 1.05 or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2022-123651 filed in Japan on Aug. 3, 2022, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball composed of three or more layers including a core, an intermediate layer, and a cover.

BACKGROUND ART

Many innovations have been made in designing golf balls with a multilayer construction, and many golf balls for professional and skilled amateur golfers with a high head speed have been developed to date. Among them, the most popular golf balls are three-piece solid golf balls composed of a core, an intermediate layer, and a cover (outermost layer). Specifically, there have been many proposals for functional three-piece solid golf balls in which a material hardness and a core surface hardness of each layer of the intermediate layer and the cover, and a surface hardness of an intermediate layer-encased sphere are optimized, and some technologies have been proposed to provide high-performance golf balls by designing a core internal hardness in various aspects while focusing on a core hardness profile occupying most of the volume of the ball.

Examples of such technical documents include the three-piece solid golf balls of JP-A 2004-97802, JP-A 2011-120898, JP-A 2016-112308, JP-A 2017-000183, JP-A 2017-000470 and JP-A 2018-183247.

However, although some of the proposed golf balls disclose a relationship between an initial velocity of each layer-encased sphere of the intermediate layer-encased sphere and the ball or a relationship between a deflection when a specific load is applied to the core and a deflection when a specific load is applied to the ball, attention on the relationship between the initial velocity of the intermediate layer-encased sphere and the initial velocity of the core and a specific gravity of the intermediate layer is not sufficient, and there is room for improvement for obtaining a golf ball with higher performance. In addition, as a golf ball for professionals or skilled amateur golfers having a high head speed, there is a demand for a golf ball having higher performance in which superior distance on full shots with a driver (W #1) and an iron, high playability in the short game, and excellent durability on repeated impact are compatible.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a golf ball that has a superior distance on shots with a driver (W #1) and an iron, has good controllability on approach shots, and imparts excellent durability on repeated impact, mainly when used by professionals or skilled amateur golfers having a high head speed.

As a result of intensive studies to achieve the above object, the present inventor has found that in a multi-piece solid golf ball including a core, an intermediate layer, and a cover, a relationship between an initial velocity of the core, an initial velocity of a sphere (intermediate layer-encased sphere) in which the core is encased with the intermediate layer, and an initial velocity of a sphere (ball) in which the intermediate layer-encased sphere is encased with the cover satisfies the following two conditions:

(initial velocity of ball)<(initial velocity of intermediate layer-encased sphere)

0.60≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).

Further, the present inventor has found that when a ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), by setting a deflection to 3.00 mm or less and a specific gravity of the intermediate layer to 1.05 or more, superior flight on shots with a driver (W #1) and an iron when used by a professional or a skilled amateur golfer with a high head speed, good controllability on approach shots, and excellent durability on repeated impact can be obtained, and thus has completed the present invention.

Accordingly, the present invention provides a multi-piece solid golf ball including a core, an intermediate layer, and a cover, wherein the core is formed of a rubber composition into a single layer or a plurality of layers, the intermediate layer and the cover are both formed of a single-layer resin composition, and a relationship between an initial velocity of the core, an initial velocity of a sphere (intermediate layer-encased sphere) obtained by encasing the core with the intermediate layer, and an initial velocity of a sphere (ball) obtained by encasing the intermediate layer-encased sphere with the cover satisfies the following two conditions:

(initial velocity of ball)<(initial velocity of intermediate layer-encased sphere)

0.60≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).

Further characteristics of the multi-piece solid golf ball are that a deflection is 3.00 mm or less when the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), and a specific gravity of the intermediate layer is 1.05 or more.

In a preferred embodiment of the golf ball according to the invention, assuming that when each sphere of the core and the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and the deflections (mm) are denoted by C (mm) and B (mm) respectively, a value of C-B is 1.45 mm or less.

In another preferred embodiment of the inventive golf ball, the following condition is satisfied:

ball surface hardness<surface hardness of intermediate layer-encased sphere>core surface hardness

(where the surface hardness of each sphere means Shore C hardness).

In yet another preferred embodiment, the resin composition of the intermediate layer contains a high-acid ionomer resin having an acid content of 16 wt % or more.

In still another preferred embodiment, the intermediate layer contains an inorganic particulate filler.

In a further preferred embodiment, a difference between a specific gravity of the cover and the specific gravity of the intermediate layer is 0.15 or less.

In a yet further preferred embodiment, the following condition is satisfied: cover thickness<intermediate layer thickness.

In a still further preferred embodiment, the core has a diameter of 36.7 to 40.1 mm and has a hardness profile in which, letting the Shore C hardness at a core center be Cc, the Shore C hardness at a midpoint M between the core center and a core surface be C_(m), the Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M be Cm−2, Cm−4, and Cm−6 respectively, the Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the center M be Cm+2, Cm+4, and Cm+6 respectively, and the Shore C hardness at the core surface be Cs, and defining surface areas A to F as follows:

surface area A: ½×2×(Cm−4−Cm−6)

surface area B: ½×2×(Cm−2−Cm−4)

surface area C: ½×2×(Cm−Cm−2)

surface area D: ½×2×(Cm+2−Cm)

surface area E: ½×2×(Cm+4−Cm+2)

surface area F: ½×2×(Cm+6−Cm+4)

the following conditions are satisfied:

(surface area E+surface area F)−(surface area A+surface area B)≥2.0.

In another preferred embodiment, the following condition is satisfied:

(surface area D+surface area E)−(surface area B+surface area C)≥2.0.

Advantageous Effects of Invention

With the golf ball according to the present invention, mainly in professional golfers and skilled amateur golfers with a high head speed, a superior distance is obtained on full shots with a driver (W #1) and an iron, a spin rate on approach shots is high, and playability in the shot game is excellent. Furthermore, the golf ball according to the present invention has good durability on repeated impact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the present invention.

FIG. 2 is a graph that uses core hardness profile data in Example 1 to describe surface areas A to F in the core hardness profile.

FIG. 3 is a graph showing the core hardness profiles in Examples 1 to 3.

FIG. 4 is a graph showing the core hardness profiles in Comparative Examples 1 to 8.

DETAILED DESCRIPTION OF THE INVENTION

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagrams.

Hereinafter, the present invention is described in more detail. A multi-piece solid golf ball according to the present invention has a core, an intermediate layer, and a cover, and an example thereof is shown in FIG. 1 , for example. A golf ball G shown in FIG. 1 has a single-layer core 1, a single-layer intermediate layer 2 encasing the core 1, and a single-layer cover 3 encasing the intermediate layer. The cover 3 is positioned at the outermost layer in the layer construction of the golf ball except for the coating layer. In addition to a single layer as shown in FIG. 1 , the core may can be formed as a plurality of layers. Note that a large number of dimples D are typically formed on the surface of the cover (outermost layer) 3 in order to improve the aerodynamic properties of the ball. In addition, although not particularly illustrated, a coating layer is typically formed on the surface of the cover 3. Hereinafter, each of the above layers is described in detail.

The core is obtained by vulcanizing a rubber composition containing a rubber material as a chief material. If the core material is not a rubber composition, the rebound of the core becomes low, and as a result, a good distance of the ball may not be achieved. This rubber composition typically contains a base rubber as a chief material, and is obtained with the inclusion of a co-crosslinking agent, a crosslinking initiator, an inert filler, an organosulfur compound, or the like.

As the base rubber, polybutadiene is preferably used. As the type of polybutadiene, a commercially available product may be used, and examples thereof include BR01, BR51, and BR730 (manufactured by JSR Corporation). The proportion of polybutadiene in the base rubber is preferably 60 wt % or more, and more preferably 80 wt % or more. In addition to the polybutadiene, other rubber components are included in the base rubber as long as the effect of the present invention is not impaired. Examples of the rubber component other than the polybutadiene include a polybutadiene other than the polybutadiene described above, and other diene rubbers such as styrene-butadiene rubber, natural rubber, isoprene rubber, and ethylene-propylene-diene rubber.

The co-crosslinking agent is an α,β-unsaturated carboxylic acid and/or a metal salt thereof. Specific examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, or the like, and in particular, acrylic acid and methacrylic acid are suitably used. The metal salt of the unsaturated carboxylic acid is not particularly limited, and examples thereof include those obtained by neutralizing the unsaturated carboxylic acid with a desired metal ion. Specific examples thereof include zinc salts and magnesium salts such as methacrylic acid and acrylic acid, and in particular, zinc acrylate is suitably used.

The unsaturated carboxylic acid and/or the metal salt thereof is typically blended in an amount of 5 parts by weight or more, preferably 10 parts by weight or more, and even more preferably 20 parts by weight or more, and the upper limit is typically 60 parts by weight or less, preferably 50 parts by weight or less, and even more preferably 45 parts by weight or less per 100 parts by weight of the base rubber. If the compounding amount is too large, the core may become too hard, giving the ball an unpleasant feel at impact, and if the compounding amount is too small, rebound may become low.

As the crosslinking initiator, an organic peroxide is suitably used. Specifically, commercially available organic peroxides can be used, and for example, Percumyl D, Perhexa C-40, Perhexa 3M (all manufactured by NOF Corporation), and Luperco 231XL (manufactured by AtoChem Corporation) can be suitably used. These may be used singly, or two or more may be used in combination. The compounding amount of the organic peroxide is preferably 0.1 parts by weight or more, more preferably 0.3 parts by weight or more, and even more preferably 0.5 parts by weight or more, and the upper limit is preferably 5 parts by weight or less, more preferably 4 parts by weight or less, even more preferably 3 parts by weight or less, and most preferably 2.5 parts by weight or less per 100 parts by weight of the base rubber. If the compounding amount is too large or too small, it may not be possible to obtain suitable feel at impact, durability, and rebound.

As a filler, for example, zinc oxide, barium sulfate, calcium carbonate, or the like may be suitably used. These may be used singly, or two or more may be used in combination. The compounding amount of the filler is preferably 1 part by weight or more, and more preferably 3 parts by weight or more per 100 parts by weight of the base rubber. In addition, an upper limit of the compounding amount is preferably 50 parts by weight or less, more preferably 40 parts by weight or less, and even more preferably 30 parts by weight or less per 100 parts by weight of the base rubber. If the compounding amount is too large or too small, it may not be possible to obtain an appropriate weight and a suitable rebound.

As an antioxidant, for example, commercially available products such as Nocrac NS-6, Nocrac NS-30, Nocrac NS-200, and Nocrac MB (all manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.) may be employed. These may be used singly, or two or more may be used in combination.

The compounding amount of the antioxidant is not particularly limited, but is preferably 0.05 parts by weight or more, and more preferably 0.1 parts by weight or more, and the upper limit is preferably 1.0 part by weight or less, more preferably 0.7 parts by weight or less, and even more preferably 0.5 parts by weight or less per 100 parts by weight of the base rubber. If the compounding amount is too large or too small, a suitable core hardness gradient cannot be obtained, and it may not be possible to obtain suitable rebound, durability, and a spin rate-lowering effect on full shots.

Furthermore, an organosulfur compound is included in the rubber composition in order to impart an excellent rebound. Specifically, it is recommended to include thiophenol, thionaphthol, halogenated thiophenol, or a metal salt thereof. More specifically, examples of the organosulfur compound include zinc salts such as pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, and pentachlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfide, dibenzylpolysulfide, dibenzoylpolysulfide, dibenzothiazoylpolysulfide, and dithiobenzoylpolysulfide. In particular, diphenyldisulfide and the zinc salt of pentachlorothiophenol is preferably used.

The organosulfur compound is blended in an amount of 5 parts by weight or less, preferably 4 parts by weight or less, more preferably 3 parts by weight or less, and most preferably 2 parts by weight or less per 100 parts by weight of the base rubber. In addition, the lower limit value of the compounding amount is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, and even more preferably 0.3 parts by weight or more. If the compounding amount is too large, the hardness becomes too soft, and if the compounding amount is too small, the rebound may not be expected to be improved.

Water may be included in the rubber composition. This water, although not particularly limited, may be distilled water or tap water, but it is particularly suitable to employ distilled water free of impurities. The compounding amount of the water included per 100 parts by weight of the base rubber. is preferably at least 0.1 parts by weight, and more preferably at least 0.2 parts by weight, and the upper limit is preferably not more than 2 parts by weight, and more preferably not more than 1.5 parts by weight.

By blending the water or a water-containing material directly into the core material, decomposition of the organic peroxide during the core formulation can be promoted. In addition, it is known that the decomposition efficiency of the organic peroxide in the core-forming rubber composition changes depending on temperature, and the decomposition efficiency increases as the temperature becomes higher than a certain temperature. If the temperature is too high, the amount of decomposed radicals becomes too large, and the radicals are recombined or deactivated. As a result, fewer radicals act effectively in crosslinking. Here, when decomposition heat is generated by the decomposition of the organic peroxide at the time of core vulcanization, a temperature near the core surface is maintained at substantially the same level as a temperature of a vulcanization mold, but the temperature around the core center is considerably higher than the mold temperature due to an accumulation of decomposition heat by the organic peroxide decomposing from the outside. If the water or a material containing water is directly included in the core, the water acts to promote the decomposition of the organic peroxide, so that the radical reactions as described above can be changed at the core center and the core surface. That is, the decomposition of the organic peroxide is further promoted near the core center, and the deactivation of radicals is further promoted, so that the amount of active radicals is further reduced, and as a result, a core can be obtained in which the crosslink densities at the core center and the core surface differ markedly.

The core can be manufactured by vulcanizing and curing the rubber composition containing the above components. For example, a molded body can be manufactured by intensively mixing the rubber composition using a mixing apparatus such as a Banbury mixer or a roll mill, subsequently compression molding or injection molding the mixture using a core mold, and curing the resulting molded body by appropriately heating it at a temperature sufficient for the organic peroxide or the co-crosslinking agent to act, such as at a temperature of 100 to 200° C., and preferably at a temperature of 140 to 180° C., for 10 to 40 minutes.

In the present invention, the core is formed as a single layer or a plurality of layers, although it is preferably formed as a single layer. If the rubber core is produced as a plurality of layers of rubber, in a case where a difference in hardness between the interfaces of these rubber layers is large, layer separation at the interface may arise when the ball is repeatedly struck, possibly leading to a loss in the initial velocity of the ball on full shots.

The diameter of the core is preferably at least 36.7 mm, more preferably at least 37.2 mm, and even more preferably at least 37.6 mm. The upper limit of the diameter of the core is preferably not more than 40.1 mm, more preferably not more than 39.0 mm, and even more preferably not more than 38.1 mm. If the diameter of the core is too small, the initial velocity of the ball may become low, or a deflection of the entire ball may become small, a spin rate of the ball on full shots rises, and an intended distance may not be attainable. On the other hand, if the diameter of the core is too large, the spin rate on full shots may rise, and the intended distance may not be attainable, or a durability to cracking on repeated impact may worsen.

The deflection (mm) when the core is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is not particularly limited, but is preferably at least 2.6 mm, more preferably at least 2.9 mm, and even more preferably at least 3.2 mm, and the upper limit thereof is preferably not more than 4.5 mm, more preferably not more than 4.3 mm, and even more preferably not more than 4.0 mm. If the deflection of the core is too small, that is, the core is too hard, the spin rate of the ball may rise excessively, and a good distance may not be achieved, or the feel at impact may be excessively hard. On the other hand, if the deflection of the core is too large, that is, the core is too soft, the ball rebound may become too low and a good distance may not be achieved, the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

Next, the core hardness profile is described. Note that the hardness of the core described below means Shore C hardness. The Shore C hardness is a hardness value measured with a Shore C durometer conforming to the ASTM D2240 standard.

A core center hardness (Cc) is preferably at least 49, more preferably at least 51, and even more preferably at least 53, and the upper limit is preferably 60 or less, more preferably 58 or less, and even more preferably 56 or less. If this value is too large, the feel at impact becomes hard, or the spin rate on full shots rises, and the intended distance may not be attainable. On the other hand, if the above value is too small, the rebound becomes low and a good distance is not achieved, or the durability to cracking on repeated impact may worsen.

A hardness (Cm−6) at a position 6 mm inward from a position M (hereinafter, also referred to as “midpoint M”) between the core center and the core surface is not particularly limited, but may be preferably at least 50, more preferably at least 52, and even more preferably at least 54, and the upper limit is also not particularly limited, and may be preferably not more than 61, more preferably not more than 59, and even more preferably not more than 57. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A hardness (Cm−4) at a position 4 mm inward from the point M (hereinafter, also referred to as “midpoint M”) between the core center and the core surface is not particularly limited, but may be preferably at least 47, preferably at least 49, and even more preferably at least 51, and the upper limit is also not particularly limited, and may be preferably not more than 58, more preferably not more than 56, and even more preferably not more than 54. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A hardness (Cm−2) at a position 2 mm inward from the midpoint M of the core is not particularly limited, but may be preferably at least 51, more preferably at least 53, and even more preferably at least 55. The upper limit is also not particularly limited, and may be preferably not more than 62, more preferably not more than 60, and even more preferably not more than 58. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc). Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A cross-sectional hardness (Cm) at the midpoint M of the core is not particularly limited, but may be preferably at least 56, more preferably at least 58, and even more preferably at least 60. In addition, the upper limit is not particularly limited, but may be preferably not more than 66, more preferably not more than 64, and even more preferably not more than 62. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A core surface hardness (Cs) is preferably at least 76, more preferably at least 78, and even more preferably at least 80. The upper limit value is preferably not more than 90, more preferably not more than 87, and even more preferably not more than 85. If this value is too large, the durability to cracking on repeated impact may worsen, or the feel at impact may be too hard. On the other hand, if the above value is too small, the rebound becomes low and a good distance is not achieved, or the spin rate on full shots rises, and the intended distance may not be attainable.

A hardness (Cm+2) at a position 2 mm outward toward the core surface (hereinafter, simply referred to as “outward”) from the midpoint M of the core toward the core surface is not particularly limited, but may be preferably at least 60, more preferably at least 62, and even more preferably at least 64. The upper limit is also not particularly limited, and may be preferably not more than 72, more preferably not more than 70, and even more preferably not more than 68. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

A hardness (Cm+4) at a position 4 mm outward from the midpoint M of the core is not particularly limited, but may be preferably at least 66, more preferably at least 68, and even more preferably at least 70. The upper limit is also not particularly limited, and may be preferably not more than 78, more preferably not more than 76, and even more preferably not more than 74. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

A hardness (Cm+6) at a position 6 mm outward from the midpoint M of the core is not particularly limited, but may be preferably at least 71, more preferably at least 73, and even more preferably at least 75. The upper limit is also not particularly limited, and may be preferably not more than 85, more preferably not more than 82, and even more preferably not more than 80. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

In the core hardness profile, the surface areas A to F defined as follows:

surface area A: ½×2×(Cm−4−Cm−6)

surface area B: ½×2×(Cm−2−Cm−4)

surface area C: ½×2×(Cm−Cm−2)

surface area D: ½×2×(Cm+2−Cm)

surface area E: ½×2×(Cm+4−Cm+2)

surface area F: ½×2×(Cm+6−Cm+4)

are characterized in that a value of (surface area E+surface area F)−(surface area A+surface area B) is preferably 2.0 or more, more preferably 4.0 or more, and more preferably 6.0 or more, and the upper limit is preferably not more than 20.0, more preferably not more than 16.0, and even more preferably not more than 10.0. If this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value becomes too small, the spin rate of the ball on full shots may rise, and the intended distance may not be attainable.

In addition, a value of (surface area D+surface area E)−(surface area B+surface area C) is preferably 2.0 or more, more preferably 3.0 or more, and even more preferably 4.0 or more, and the upper limit is preferably not more than 20.0, more preferably not more than 16.0, and even more preferably not more than 10.0. If this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value becomes too small, the spin rate of the ball on full shots may rise, and the intended distance may not be attainable.

The surface areas A to F preferably satisfy the following conditions:

surface area A<surface area C<(surface area E+surface area F) and surface area B<surface area C<(surface area E+surface area F)

and more preferably satisfy the following conditions:

surface area A<surface area C<surface area D<(surface area E+surface area F) and surface area B<surface area C<surface area D<(surface area E+surface area F)

and even more preferably satisfy the following condition:

surface area A<surface area B<surface area C<surface area D<(surface area E+surface area F).

If these relationships are not satisfied, the spin rate of the ball on full shots rises, and the intended distance may not be attainable.

FIG. 2 shows a graph describing the surface areas A to F using the core hardness profile data of Example 1. In this way, the surface areas A to F are surface areas of each triangle whose base is a difference between each specific distance and whose height is a difference in hardness between each position at these specific distances.

An initial velocity of the core is preferably at least 75.8 m/s, more preferably at least 76.3 m/s, and even more preferably at least 76.7 m/s. The upper limit is preferably not more than 78.0 m/s, more preferably not more than 77.5 m/s, and even more preferably not more than 77.0 m/s. If the initial velocity value is too high, the initial velocity of the ball becomes too fast, and it may be against the rules. On the other hand, if the initial velocity of the core becomes too low, the ball rebound on full shots may become low, or the spin rate of the ball rises, and the intended distance may not be attainable. The value of the initial velocity in this case is a numerical value measured by a device for measuring a coefficient of restitution (COR) (Golf Ball Testing Machine) of the same type as the R&A. Specifically, a Golf Ball Testing Machine manufactured by Hye Precision USA is used. As a condition, at the time of measurement, an air pressure is changed in four stages and measured, a relational expression between the incident velocity and the COR is constructed, and the initial velocity at an incident velocity of 43.83 m/s is determined from the relational expression. Note that for a measurement environment of the Golf Ball Testing Machine, a ball temperature-controlled for three hours or more in a thermostatic bath adjusted to 23.9±1° C. is used, and measurement is performed at a room temperature of 23.9±2° C.

Next, the intermediate layer is described.

The intermediate layer has a material hardness on the Shore C hardness scale which, although not particularly limited, is preferably at least 90, more preferably at least 92 and even more preferably at least 93, but is preferably not more than 100, more preferably not more than 98 and even more preferably not more than 96. The material hardness on the Shore D hardness scale is preferably at least 64, more preferably at least 66 and even more preferably at least 68, but is preferably not more than 75, more preferably not more than 72 and even more preferably not more than 70.

The sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) has a surface hardness which, on the Shore C hardness scale, is preferably at least 95, more preferably at least 96, and even more preferably at least 97. The upper limit is preferably not more than 100, more preferably not more than 99, and even more preferably not more than 98. The surface hardness on the Shore D hardness scale is preferably at least 68, more preferably at least 69, and even more preferably at least 70. The upper limit is preferably not more than 78, more preferably not more than 75, and even more preferably not more than 72.

If the material hardness and the surface hardness of the intermediate layer are too soft in comparison with the above ranges, the spin rate on full shots may rise excessively so that the distance may not be increased, or the initial velocity of the ball may become low so that the distance may not be increased. On the other hand, if the material hardness and the surface hardness of the intermediate layer are too hard in comparison with the above ranges, the durability to cracking on repeated impact may worsen, or the feel at impact on shots with a putter or on short approaches may become too hard.

The intermediate layer has a thickness which is preferably at least 1.00 mm, more preferably at least 1.25 mm, and even more preferably at least 1.45 mm. The intermediate layer thickness has an upper limit that is preferably not more than 1.80 mm, more preferably not more than 1.65 mm, and even more preferably not more than 1.55 mm. It is preferable for the intermediate layer to be thicker than the subsequently described cover. When the intermediate layer thickness falls outside of the above range or the intermediate layer is thinner than the cover, the ball spin rate-lowering effect on shots with a driver (W #1) may be inadequate, resulting in a poor distance. Also, when the intermediate layer is too thin, the durability to cracking on repeated impact and the low-temperature durability may worsen.

The value obtained by subtracting the cover thickness from the intermediate layer thickness is preferably larger than 0 mm, more preferably 0.3 mm or more, and even more preferably 0.5 mm or more. The upper limit is preferably not more than 1.5 mm, more preferably not more than 1.0 mm, and even more preferably not more than 0.7 mm. When this value falls outside of the above range, the spin rate of the ball on full shots may rise or the initial velocity on shots may become low, as a result of which the intended distance may be unattainable. When this value is too small, the durability to cracking on repeated impact may worsen.

As a material of the intermediate layer, it is suitable to employ an ionomer resin as a chief material.

The ionomer resin material suitably contains a high-acid ionomer resin having an unsaturated carboxylic acid content (also referred to as “acid content”) of 16 wt % or more.

The amount of high-acid ionomer resin included per 100 wt % of the resin material is preferably at least 20 wt %, more preferably at least 50 wt %, and even more preferably at least 60 wt %. The upper limit is preferably 100 wt % or less, more preferably 90 wt % or less, and even more preferably 85 wt % or less. When the content of this high-acid ionomer resin is too low, the spin rate of the ball on full shots may rise and a good distance may not be attained. On the other hand, when the content of this high-acid ionomer resin is too high, the durability to repeated impact may worsen.

In addition, if an ionomer resin is employed as the chief material, an aspect that uses in admixture a zinc-neutralized ionomer resin and a sodium-neutralized ionomer resin as the chief materials is desirable. The blending ratio in terms of zinc-neutralized ionomer resin/sodium-neutralized ionomer resin (weight ratio) is from 5/95 to 95/5, preferably from 10/90 to 90/10, and more preferably from 15/85 to 85/15. If the zinc-neutralized ionomer and the sodium-neutralized ionomer are not included in this ratio, the rebound may become too low to obtain a desired flight, the durability to cracking on repeated impact at room temperature may worsen, and the durability to cracking at a low temperature (below zero) may worsen.

In the intermediate layer material, an optional additive may be appropriately included depending on the intended use. For example, various additives such as a pigment, a dispersant, an antioxidant, an ultraviolet absorber, and a light stabilizer can be included. If these additives are included, the compounding amount thereof is preferably 0.1 parts by weight or more, and more preferably 0.5 parts by weight or more, and an upper limit thereof is preferably 10 parts by weight or less, and more preferably 4 parts by weight or less per 100 parts by weight of the base resin.

For the intermediate layer material, it is suitable to abrade the surface of the intermediate layer in order to increase the degree of adhesion to a polyurethane suitably used in a cover material described later. Further, it is preferable that a primer (adhesive agent) is applied to the surface of the intermediate layer after the abrasion treatment, or an adhesion reinforcing agent is added to the intermediate layer material.

The material of the intermediate layer can contain an inorganic particulate filler. This inorganic particulate filler is not particularly limited, but zinc oxide, barium sulfate, titanium dioxide, or the like can be appropriately used. Barium sulfate can be preferably used, and particularly preferably, precipitated barium sulfate can be suitably used from the viewpoint of excellent durability to cracking on repeated impact.

The mean particle size of the inorganic particulate filler is not particularly limited, but is preferably 0.01 to 100 μm, and more preferably 0.1 to 10 μm. If the mean particle size of the inorganic particulate filler is too small or too large, dispersibility during material preparation may be deteriorated. Note that the mean particle size means a particle size measured by dispersing the particles in an aqueous solution together with an appropriate dispersant and measuring the particles with a particle size distribution measuring device.

The content of the inorganic filler is not particularly limited, although the content is preferably set to at least 0 part by weight, more preferably at least 10 parts by weight, and even more preferably at least 15 parts, per 100 parts by weight of the base resin of the intermediate layer material. Although there is no particular upper limit, the content is preferably not more than 50 parts by weight, more preferably not more than 40 parts by weight, and even more preferably not more than 30 parts by weight. At an inorganic filler content that is too low, the durability to cracking on repeated impact may worsen. On the other hand, at an inorganic filler content that is too high, the ball rebound may decrease or the spin rate of the ball on full shots may rise, as a result of which the intended distance may not be achieved.

A specific gravity of the intermediate layer is preferably 1.05 or more, more preferably 1.07 or more, and even more preferably 1.09 or more, and the upper limit value thereof is preferably 1.25 or less, more preferably 1.20 or less, and even more preferably 1.15 or less. If the specific gravity of the intermediate layer is too small, the durability to cracking on repeated impact may worsen. On the other hand, if the specific gravity of the intermediate layer is too large, the ball rebound becomes low, or the spin rate of the ball on full shots rises, and the intended distance may not be attained.

The intermediate layer-encased sphere has an initial velocity which is preferably at least 77.0 m/s, more preferably at least 77.3 m/s, and even more preferably at least 77.5 m/s. The upper limit is preferably not more than 78.5 m/s, more preferably not more than 78.2 m/s, and even more preferably not more than 77.9 m/s. When this initial velocity is too high, the ball initial velocity may become too high and end up falling outside the range specified in the Rules of Golf. If the initial velocity value is too high, the initial velocity of the ball becomes too fast, and it may be against the rules. On the other hand, if the initial velocity is too low, the ball rebound may become low on full shots, or the spin rate rises, and the intended distance may not be attainable. The initial velocity in this case is the same as the device and conditions used in the measurement of the initial velocity of the core described above.

Next, the cover is described.

The cover has a material hardness on the Shore C hardness scale which, although not particularly limited, is preferably at least 50, more preferably at least 57 and even more preferably at least 63, but is preferably not more than 80, more preferably not more than 74 and even more preferably not more than 70. The material hardness on the Shore D hardness scale is preferably at least 30, more preferably at least 35 and even more preferably at least 40, but is preferably not more than 53, more preferably not more than 50 and even more preferably not more than 47.

The sphere obtained by encasing the intermediate layer-encased sphere with the cover—that is, the ball—has a surface hardness which, on the Shore C hardness scale, is preferably at least 73, more preferably at least 78 and even more preferably at least 83, but is preferably not more than 95, more preferably not more than 92 and even more preferably not more than 90. The surface hardness on the Shore D hardness scale is preferably at least 50, more preferably at least 53 and even more preferably at least 56, but is preferably not more than 70, more preferably not more than 65 and even more preferably not more than 60.

If the material hardness and the surface hardness of the cover are too soft in comparison with the above ranges, the spin rate on full shots may rise excessively, and the distance may not be increased. On the other hand, if the material hardness and the surface hardness of the cover are too hard in comparison with the above ranges, the ball may not be fully receptive to spin on approach shots, or a scuff resistance may worsen.

The cover has a thickness of preferably at least 0.3 mm, more preferably at least 0.45 mm, and even more preferably at least 0.6 mm. The upper limit in the cover thickness is preferably not more than 1.2 mm, more preferably not more than 0.9 mm, and even more preferably not more than 0.8 mm. When the cover is too thick, the rebound of the ball on full shots may be inadequate or the spin rate may rise, as a result of which a good distance may not be achieved. On the other hand, when the cover is too thin, the scuff resistance may worsen or the ball may not be receptive to spin on approach shots and may thus lack sufficient controllability.

As a cover material, various types of thermoplastic resin used as a cover material in golf balls can be used, but it is suitable to use a resin material composed primarily of a thermoplastic polyurethane from the viewpoints of spin controllability in the short game and scuff resistance. That is, the cover is suitably formed of a resin blend containing (I) a thermoplastic polyurethane and (II) a polyisocyanate compound as principal components.

The total weight of the components (I) and (II) is recommended to be 60% or more, and more preferably 70% or more with respect to the total amount of the resin composition of the cover. The components (I) and (II) are described in detail below.

Describing the thermoplastic polyurethane (I), the construction of the thermoplastic polyurethane includes a soft segment composed of a polymeric polyol (polymeric glycol), which is a long-chain polyol, and a hard segment composed of a chain extender and a polyisocyanate compound. Here, as the long-chain polyol serving as a starting material, any of those hitherto used in the art related to thermoplastic polyurethane can be used, and are not particularly limited, and examples thereof can include polyester polyol, polyether polyol, polycarbonate polyol, polyester polycarbonate polyol, polyolefin polyol, conjugated diene polymer-based polyol, castor oil-based polyol, silicone-based polyol, and vinyl polymer-based polyol. These long-chain polyols may be used singly, or two or more may be used in combination. Among them, a polyether polyol is preferable from the viewpoint that a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties can be synthesized.

As the chain extender, those hitherto used in the art related to thermoplastic polyurethanes can be suitably used, and for example, a low-molecular-weight compound having on the molecule two or more active hydrogen atoms capable of reacting with an isocyanate group and having a molecular weight of 400 or less is preferable. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, or the like. Among them, the chain extender is preferably an aliphatic diol having from 2 to 12 carbon atoms, and is more preferably 1,4-butylene glycol.

As the polyisocyanate compound, those hitherto used in the art related to thermoplastic polyurethane can be suitably used, and are not particularly limited. Specifically, one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate (or) 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate can be used. However, it may be difficult to control a crosslinking reaction during injection molding depending on the type of isocyanate. In the present invention, 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferable from the viewpoint of providing a balance between stability during production and the physical properties to be manifested.

As specific examples of the thermoplastic polyurethane serving as the component (I), commercially available products can be used such as Pandex T-8295, Pandex T-8290, and Pandex T-8260 (all manufactured by DIC Covestro Polymer, Ltd.).

Although not an essential component, a thermoplastic elastomer other than the thermoplastic polyurethane can be included as a separate component (III) with the components (I) and (II). By including the component (III) in the resin blend, a flowability of the resin blend can be further improved, and various physical properties required of the golf ball cover material can be increased, such as rebound and scuff resistance.

A compositional ratio of the components (I), (II), and (III) is not particularly limited, but in order to sufficiently and effectively exhibit the advantageous effects of the present invention, the compositional ratio (I):(II):(III) is preferably in the weight ratio range of from 100:2:50 to 100:50:0, and more preferably from 100:2:50 to 100:30:8.

Furthermore, various additives other than the components constituting the thermoplastic polyurethane can be included in the resin blend as necessary, and for example, a pigment, a dispersant, an antioxidant, a light stabilizer, an ultraviolet absorber, an internal mold lubricant, or the like can be appropriately included.

The cover has a specific gravity which, although not particularly limited, is preferably at least 1.00, more preferably at least 1.03, and even more preferably at least 1.06. The upper limit is preferably not more than 1.20, more preferably not more than 1.17, and even more preferably not more than 1.14. When the cover specific gravity is lower than the above range, the ratio of low specific gravity materials such as ionomer blended into the cover made chiefly of urethane ends up becoming high, as a result of which the scuff resistance may worsen. On the other hand, when the cover specific gravity is too high, the amount of filler added is high and the rebound may become too low, as a result of which the intended distance may be unattainable.

The manufacture of a multi-piece solid golf ball in which the above-described core, intermediate layer, and cover (outermost layer) are formed as successive layers can be performed by a customary method such as a known injection molding process. For example, an intermediate layer material is injected around the core in an injection mold to obtain an intermediate layer-encased sphere, and finally, a cover material, which is the outermost layer, is injection molded to obtain a multi-piece golf ball. In addition, it is also possible to produce a golf ball by preparing two half-cups pre-molded into hemispherical shapes, enclosing the core and the intermediate layer-encased sphere within the two half cups, and molding the core and the intermediate layer-encased sphere under applied heat and pressure.

The golf ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which is preferably at least 2.3 mm, more preferably at least 2.4 mm, and even more preferably at least 2.5 mm. The deflection upper limit is preferably not more than 3.0 mm, more preferably not more than 2.9 mm, and even more preferably not more than 2.8 mm. When the golf ball deflection is too small, i.e., when the ball is too hard, the spin rate may rise excessively, resulting in a poor flight, or the feel at impact may be too hard. On the other hand, when the deflection is too large, i.e., when the ball is too soft, the ball rebound may be too low, resulting in a poor flight, the feel at impact may be too soft, or the durability to cracking under repeated impact may worsen.

The ball has an initial velocity which is preferably at least 76.8 m/s, more preferably at least 77.0 m/s, and even more preferably at least 77.2 m/s. The upper limit is preferably not more than 77.724 m/s. A ball initial velocity that is too high may fall outside the range specified in the Rules of Golf. On the other hand, when the ball initial velocity is too low, the ball may not travel well on full shots. The initial velocity in this case is measured with the same device and under the same conditions as described above for measurement of the initial velocities of the core and the intermediate layer-encased sphere.

[Relationships Between Surface Hardnesses of Each Sphere]

Expressed on the Shore C scale, a value obtained by subtracting the core surface hardness from the surface hardness of the intermediate layer-encased sphere is preferably 5 or more, more preferably 10 or more, and even more preferably 14 or more, and the upper limit is preferably 27 or less, more preferably 22 or less, and even more preferably 17 or less. If there is a deviation from the above ranges, the spin rate of the ball on full shots may rise, and the intended distance may not be attainable.

Expressed on the Shore C scale, a value obtained by subtracting the core center hardness from the surface hardness of the intermediate layer-encased sphere is preferably 40 or more, more preferably 41 or more, and even more preferably 42 or more, and the upper limit is preferably 53 or less, more preferably 48 or less, and even more preferably 45 or less. If the above value is too small, the spin rate of the ball may rise on full shots, and the intended distance may not be attained. On the other hand, if the above value is too large, the durability to cracking on repeated impact may worsen, or the actual initial velocity on shots becomes lower, and the intended distance may not be attainable.

Expressed on the Shore C scale, a value obtained by subtracting the surface hardness of the ball from the surface hardness of the intermediate layer-encased sphere is preferably 2 or more, more preferably 4 or more, and even more preferably 6 or more, and the upper limit is preferably 25 or less, more preferably 17 or less, and even more preferably 14 or less. If the above value is too small, controllability in the short game may worsen. On the other hand, if the above value is too large, the spin rate on full shots may rise, and the intended distance may not be attainable.

[Initial Velocity Relationships of Each Sphere]

It is critical for the relationship between the initial velocity of the core, the initial velocity of the sphere (intermediate layer-encased sphere) in which the core is encased with the intermediate layer, and the initial velocity of the sphere (ball) in which the intermediate layer-encased sphere is encased with the cover to satisfy the following two conditions:

(initial velocity of ball)<(initial velocity of intermediate layer-encased sphere)

0.60≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).

By optimizing the initial velocity relationship of these layers, it is possible to obtain a desired distance by suppressing the spin rate on full shots, and the durability to cracking on repeated impact is improved.

The value obtained by subtracting the initial velocity of the ball from the initial velocity of the intermediate layer-encased sphere is greater than 0 m/s, preferably 0.10 m/s or more, and more preferably 0.30 m/s or more. The upper limit is preferably not more than 1.00 m/s, more preferably not more than 0.70 m/s, and even more preferably not more than 0.50 m/s. If this value is too large, the spin rate of the ball rises on full shots, the actual initial velocity on shots becomes low, or the like, and the intended distance may not be attainable. On the other hand, if this value is too small due to the cover, the cover becomes hard and the ball is not receptive to spin in the short game, or the durability on repeated impact may be inferior. In addition, if this value is small due to the intermediate layer, the spin rate of the ball rises on full shots, and the intended distance may not be attainable.

The value obtained by subtracting the initial velocity of the core from the initial velocity of the intermediate layer-encased sphere is preferably 0.60 m/s or more, more preferably 0.70 m/s or more, and even more preferably 0.75 m/s or more, and the upper limit is preferably 1.00 m/s or less, more preferably 0.95 m/s or less, and even more preferably 0.90 m/s or less. If this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value is too small, the spin rate rises on full shots, and the intended distance may not be attainable.

[Specific Gravity Relationship Between Intermediate Layer and Cover]

It is recommended that a difference in the specific gravity of each layer between the specific gravity of the intermediate layer and the specific gravity of the cover is typically within ±0.15, preferably within ±0.10, and more preferably within ±0.05. That is, a value of (specific gravity of cover)−(specific gravity of intermediate layer material) is typically −0.15 or more, preferably −0.10 or more, and more preferably −0.05 or more, and the upper limit is typically 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. If the difference in specific gravity between these layers is too large, in a case where the intermediate layer material and/or the cover material cannot be molded on a completely concentric circle with these layers and with the layers located inside these layers and is eccentric, the ball hit with a putter may greatly wobble to the left or right.

[Specific Gravity Relationship Between Intermediate Layer and Core]

It is recommended that a difference in specific gravity of each layer between the specific gravity of the intermediate layer and the specific gravity of the core is typically within ±0.15, preferably within ±0.10, and more preferably within ±0.05. That is, the value of (specific gravity of intermediate layer)−(specific gravity of core) is typically −0.15 or more, preferably −0.10 or more, and more preferably −0.05 or more, and the upper limit value is typically 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. If the difference in specific gravity between these layers is too large, in a case where the intermediate layer material cannot be molded on a completely concentric circle with the core layer and is eccentric, the ball hit with a putter may greatly wobble to the left or right.

[Specific Gravity Relationship Between Core and Cover]

It is recommended that a difference in specific gravity of each layer between the specific gravity of the core and the specific gravity of the cover is typically within ±0.15, preferably within ±0.10, and more preferably within ±0.05. That is, the value of (specific gravity of cover)−(specific gravity of core) is typically −0.15 or more, preferably −0.10 or more, and more preferably −0.05 or more, and an upper limit value thereof is typically 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. If the difference in specific gravity between these layers is too large, in a case where the cover material cannot be molded on a completely concentric circle with the core layer or the intermediate layer-encased sphere and is eccentric, the ball hit with a putter may greatly wobble to the left or right.

[Core Diameter and Ball Diameter]

A relationship between the core diameter and the ball diameter, that is, a value of (core diameter)/(ball diameter) is preferably 0.860 or more, more preferably 0.870 or more, and even more preferably 0.880 or more. The upper limit is preferably 0.940 or less, more preferably 0.910 or less, and even more preferably 0.895 or less. If this value is too small, the initial velocity of the ball becomes low, or the deflection of the entire ball becomes small and the ball becomes hard, the spin rate of the ball on full shots rises, and the intended distance may not be attainable. On the other hand, if the above value is too large, the spin rate of the ball on full shots rises, and the intended distance cannot be attained, or the durability to cracking on repeated impact may worsen.

[Difference in Deflection Between Core and Ball]

When each sphere of the core and the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and the deflections (mm) are denoted by C (mm) and B (mm) respectively, a value of C-B is preferably 1.45 mm or less, more preferably 1.37 mm or less, and even more preferably 1.30 mm or less. The lower limit is preferably at least 0.40 mm, more preferably at least 0.80 mm, and even more preferably at least 1.10 mm. If this value is too large, the actual initial velocity when struck with a driver (W #1) may become low, and the intended distance may not be attainable. On the other hand, if this value is too small, the durability to cracking on repeated impact may worsen.

Numerous dimples may be formed on the outside surface of the cover. The number of dimples arranged on the surface of the cover is not particularly limited, but is preferably 250 or more, preferably 300 or more, and more preferably 320 or more, and an upper limit thereof is preferably 380 or less, more preferably 350 or less, and even more preferably 340 or less. If the number of dimples is larger than the above range, a ball trajectory may become lower, and a distance traveled by the ball may decrease. On the other hand, if the number of dimples decreases, the ball trajectory may become higher, and the distance traveled by the ball may not increase.

As for the shape of the dimples, one type or a combination of two or more types such as a circular shape, various polygonal shapes, a dewdrop shape, and other oval shapes can be appropriately used. For example, if circular dimples are used, the diameter can be about 2.5 mm or more and 6.5 mm or less, and the depth may be 0.08 mm or more and 0.30 mm or less.

A dimple coverage ratio of the dimples on the spherical surface of the golf ball, specifically, a ratio (SR value) of a sum of the individual dimple surface areas, each defined by a flat plane circumscribed by an edge of a dimple, to a ball spherical surface area on the assumption that the ball has no dimples is desirably 70% or more and 90% or less from the viewpoint of sufficiently exhibiting aerodynamic properties. In addition, a value V0 obtained by dividing the spatial volume of the dimples below the flat plane circumscribed by the edge of each dimple by a volume of a cylinder whose base is the flat plane and whose height is a maximum depth of the dimple from the base is suitably 0.35 or more and 0.80 or less from the viewpoint of optimizing the ball trajectory. Furthermore, a VR value of a sum of the volumes of the individual dimples, formed below the flat plane circumscribed by the edge of a dimple, to a ball spherical volume on the assumption that the ball has no dimples is preferably 0.6% or more and 1.0% or less. If there is a deviation from the ranges of each numerical value described above, the resulting trajectory may not enable a good distance to be attained, and the ball may fail to travel a sufficiently satisfactory distance.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 3, Comparative Examples 1 to 8

Formation of Core

In Example 2 and Comparative Examples 1 to 6, a rubber composition of each Example shown in Table 1 was prepared, and then vulcanization molding was performed under vulcanization conditions according to each Example shown in Table 1 to produce a solid core.

In Examples 1 and 3 and Comparative Examples 7 and 8, cores are produced based on the formulations in Table 1 in the same manner as described above.

TABLE 1 Example Comparative Example 1 2 3 1 2 3 4 5 6 7 8 Core Polybutadiene A 100 formulation Polybutadiene B 100 100 100 100 100 100 100 100 100 (pbw) Polybutadiene C 100 Zinc acrylate 40.5 38.5 37.5 34.5 32.5 36.5 34.5 34.5 32.5 23.5 23.0 Organic peroxide A 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.3 0.6 Organic peroxide B 0.3 1.2 Water 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Antioxidant A 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Antioxidant B 0.3 Zinc stearate 2 3 Zinc oxide 9.0 9.9 10.3 11.6 12.5 18.0 18.8 11.6 12.5 29.3 29.1 Zinc salt of 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.6 0.6 pentachlorothiophenol Vulcanization Temperature (° C.) 152 152 152 152 152 152 152 152 152 158 158 conditions Time (min) 19 19 19 19 19 19 19 19 19 14 13

Details of the above formulations are as follows.

-   -   Polybutadiene A: Trade name “BR01” (manufactured by JSR         Corporation) Polybutadiene B: Trade name “BR730” (manufactured         by JSR Corporation)     -   Polybutadiene C: Trade name “T0700” (manufactured by JSR         Corporation)     -   Zinc acrylate: Trade name “ZN-DA85 S” (manufactured by Nippon         Shokubai Co., Ltd.)     -   Organic peroxide A: Dicumyl peroxide, trade name “Percumyl D”         (manufactured by NOF Corporation)     -   Organic peroxide B: A mixture of         1,1-di(t-butylperoxy)cyclohexane and silica, trade name “Perhexa         C-40” (manufactured by NOF Corporation)     -   Water: Pure water (manufactured by Seiki Co., Ltd.)     -   Antioxidant A: 2,2-methylenebis(4-methyl-6-butylphenol), trade         name “Nocrac NS-6” (manufactured by Ouchi Shinko Chemical         Industrial Co., Ltd.)     -   Antioxidant B: 2-mercaptobenzimidazole, trade name “Nocrac MB”         (manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.)     -   Zinc stearate: Trade name “Zinc stearate GP” (manufactured by         NOF Corporation)     -   Zinc oxide: Trade name “Grade 3 Zinc Oxide” (manufactured by         Sakai Chemical Industry Co., Ltd.)     -   Zinc salt of pentachlorothiophenol: Manufactured by Wako Pure         Chemical Industries, Ltd.

Formation of Intermediate Layer and Cover (Outermost Layer)

Next, in Example 2 and Comparative Examples 1 to 6, the intermediate layer was formed by injection molding the resin materials No. 1 to No. 3 of the intermediate layer shown in Table 2 around the core surface using an injection mold. Subsequently, the cover was formed by injection molding the resin material No. 8 of the cover (outermost layer) shown in Table 2 around the intermediate layer-encased sphere using a separate injection mold. At this time, a predetermined large number of dimples common to all Examples and Comparative Examples were formed on the surface of the cover.

In Examples 1 and 3 and Comparative Examples 7 and 8, the intermediate layer is formed around the core surface by injection molding using the injection mold and the resin material No. 2, No. 4, or No. 5 of the intermediate layer shown in Table 2. Next, using a separate injection mold, injection molding is performed with the resin material No. 7 or No. 8 of the cover (outermost layer) shown in Table 2 around the intermediate layer-encased sphere to form the cover. At this time, a predetermined large number of dimples common to all Examples and Comparative Examples are formed on the surface of the cover.

TABLE 2 Acid content (wt %) Metal type No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 Himilan 1605 15 Na 50 44 30 Himilan 1855 10 Zn 30 Himilan 1557 12 Zn 15 15 15 Himilan 1706 15 Zn 35 AM 7318 18 Na 85 85 60 AM 7327 7 Zn 40 40 AN 4319 8 Un- 42 100 neutralized AN 4221C 12 Un- 14 neutralized Magnesium oxide 0.9 1.9 Magnesium stearate 33.6 70 Titanium oxide 4 4 3 Barium sulfate 20 20 Trimethylolpropane 1.1 1.1 1.1 TPU 100

Details of the blending components in the above table are as follows.

-   -   Trade names of chief materials mentioned in the table are as         follows.     -   “Himilan 1605”, “Himilan 1855”, “Himilan 1557”, “Himilan 1706”,         “AM7318”, and “AM7327” ionomer resins manufactured by Dow-Mitsui         Polychemicals Co., Ltd. “Nucrel AN4319” and “Nucrel AN4221C”         manufactured by Dow-Mitsui Polychemicals Co., Ltd.     -   “Kyowamag MF-150” magnesium oxide manufactured by Kyowa Chemical         Industry Co., Ltd.     -   “Precipitated Barium Sulfate 300” barium sulfate manufactured by         Sakai Chemical Industry Co., Ltd.     -   “Trimethylolpropane” (TMP) manufactured by Tokyo Chemical         Industry Co., Ltd.     -   “Pandex” ether-type thermoplastic polyurethane (TPU), material         hardness (Shore D) 46, manufactured by DIC Covestro Polymer Ltd.

For each resulting golf ball, various physical properties such as internal hardnesses at various positions of the core, the diameters of the core and each layer-encased sphere, thicknesses and material hardnesses of each layer, surface hardnesses of each layer-encased sphere, and initial velocities of each layer-encased sphere are evaluated by the following methods, and are shown in Tables 3 and 4.

Core Hardness Profile

The core surface is spherical, but an indenter of a durometer is set substantially perpendicular to the spherical core surface, and a core surface hardness expressed on the Shore C scale is measured in accordance with ASTM D2240. With respect to the core center and a predetermined position of the core, the core is cut into hemispheres to obtain a flat cross-section, the hardness is measured by perpendicularly pressing the indenter of the durometer against the center portion and the predetermined positions shown in Table 3, and the hardness at the center and each position are shown as Shore C hardness values. For the measurement of the hardness, a P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. equipped with a Shore C durometer is used. For the hardness value, a maximum value is read. All measurements are carried out in an environment of 23±2° C. Note that the numerical values in the table are Shore C hardness values.

In addition, in the core hardness profile, letting Cc be the Shore C hardness at the core center, Cm be the Shore C hardness at the midpoint M between the core center and the core surface, Cm−2, Cm−4, and Cm−6 be respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M, Cm+2, Cm+4, and Cm+6 be respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the center M, and Cs be the Shore C hardness at the core surface, the surface areas A to F are calculated as follows:

surface area A: ½×2×(Cm−4−Cm−6)

surface area B: ½×2×(Cm−2−Cm−4)

surface area C: ½×2×(Cm−Cm−2)

surface area D: ½×2×(Cm+2−Cm)

surface area E: ½×2×(Cm+4−Cm+2)

surface area F: ½×2×(Cm+6−Cm+4)

and the values of the following six expressions are determined:

surface areas: A+B  (1)

surface areas: B+C  (2)

surface areas: D+E  (3)

surface areas: E+F  (4)

(surface areas: E+F)−(surface areas: A+B)  (5)

(surface areas: D+E)−(surface areas: B+C)  (6)

The surface areas A to F in the core hardness profile are described in FIG. 2 , which shows a graph that illustrates surface areas A to F using the core hardness profile data from Example 1.

In addition, FIGS. 3 and 4 show graphs of core hardness profiles for Examples 1 to 3 and Comparative Examples 1 to 8.

Diameters of Core and of Intermediate Layer-Encased Sphere

At a temperature adjusted to 23.9±1° C. for at least three hours or more in a thermostatic bath, five random places on the surface are measured in a room with a temperature of 23.9±2° C., and, using an average value of these measurements as a measured value of each sphere, an average value for the diameter of 10 such spheres is determined.

Ball Diameter

At a temperature adjusted to 23.9±1° C. for at least three hours or more in a thermostatic bath, a diameter at 15 random dimple-free places is measured in a room at a temperature of 23.9±2° C., and, using an average value of these measurements as a measured value of one ball, an average value for the diameter of 10 balls is determined.

Deflections of Core, Intermediate Layer-Encased Sphere, and Ball

Each subject layer-encased sphere is placed on a hard plate, and a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is measured. Note that the deflection in each case is a measurement value measured in a room at a temperature of 23.9±2° C. after temperature adjustment to 23.9±1° C. for at least three hours or more in a thermostatic bath. A pressing speed of the core, the layer-encased sphere of each layer, or a head that compresses the ball is set to 10 mm/s.

Material Hardnesses of Intermediate Layer and Cover (Shore C and Shore D Hardnesses)

The resin material of each layer is molded into a sheet having a thickness of 2 mm and left at a temperature of 23±2° C. for two weeks. At the time of measurement, three such sheets are stacked together. The Shore C hardness and the Shore D hardness are each measured with a Shore C durometer and a Shore D durometer conforming to the ASTM D2240 standard. For the measurement of the hardness, the P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. to which a Shore C durometer or a Shore D durometer is mounted is used. For the hardness value, a maximum value is read. The measurement method is in accordance with the ASTM D2240 standard.

Surface Hardnesses of Intermediate Layer-Encased Sphere and of Ball

A measurement is performed by perpendicularly pressing the indenter against the surface of each sphere. Note that a surface hardness of a ball (cover) is a measured value at a dimple-free area (land) on the surface of the ball. The Shore C hardness and the Shore D hardness are each measured with a Shore C durometer and a Shore D durometer conforming to the ASTM D2240 standard. For the measurement of the hardness, the P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. to which a Shore C durometer or a Shore D durometer is mounted is used. For the hardness value, a maximum value is read. The measurement method is in accordance with the ASTM D2240 standard.

Initial Velocity of Each Sphere

The measurement principle for measuring an initial velocity of each sphere using the device for measuring COR manufactured by Hye Precision Products of the same type as the R&A is shown below.

An air pressure is changed to four stages of 35.5 psi, 36.5 psi, 39.5 psi, and 40.5 psi, and a ball is fired at four stages of incident velocity by respective air pressures, collided with a barrier, and its COR is measured. That is, a correlation equation between the incident velocity and the COR is created by changing the air pressure in four stages. Similarly, a correlation equation between the incident velocity and a contact time is created.

Then, from these correlation equations, the COR (coefficient of restitution) and the contact time (μs) at an incident velocity of 43.83 m/s are determined and substituted into the following initial velocity conversion equation to calculate an initial velocity of each sphere.

IV=136.8+136.3e+0.019tc

[Here, e is a coefficient of restitution, and tc is a contact time (μs) at a collision speed of 143.8 ft/s (43.83 m/s).] In the initial velocity measurement of each sphere, the barrel inner diameter used is 39.88 mm for the cores of Examples 1 to 3 and Comparative Examples 1 to 5, 38.23 mm for the cores of Comparative Examples 6 to 8, 41.53 mm for the intermediate layer-encased spheres of all Examples, and 43.18 mm for the balls of all Examples.

TABLE 3 Example Comparative Example 1 2 3 1 2 3 4 5 6 7 8 Construction 3P 3P 3P 3P 3P 3P 3P 3P 3P 3P 3P Core Outer diameter (mm) 38.04 38.06 38.06 38.04 37.99 38.04 38.04 38.04 37.99 37.37 37.34 Weight (g) 32.6 32.7 32.6 32.6 32.5 33.8 33.9 32.6 32.5 32.8 32.8 Specific gravity 1.13 1.13 1.13 1.13 1.13 1.17 1.17 1.13 1.13 1.20 1.20 Deflection (mm) 3.74 4.05 4.27 4.69 4.96 4.23 4.66 4.69 4.96 4.42 3.49 Initial velocity (m/s) 76.95 76.85 76.73 76.54 76.49 76.48 76.43 76.54 76.49 76.43 76.53 Cs (Shore C) 84.4 81.8 80.7 78.8 75.1 81.1 78.2 78.8 75.1 72.7 81.8 Cm + 6 (Shore C) 79.3 76.4 74.6 71.3 68.3 75.4 72.0 71.3 68.3 66.9 75.4 Cm + 4 (Shore C) 73.3 71.3 69.9 67.3 65.3 71.0 68.1 67.3 65.3 66.6 73.7 Cm + 2 (Shore C) 66.7 65.6 64.4 62.6 61.7 65.5 63.9 62.6 61.7 66.0 71.7 Cm (Shore C) 61.4 60.7 59.5 58.0 57.6 60.2 59.8 58.0 57.6 64.8 69.9 Cm − 2 (Shore C) 57.7 56.5 55.8 54.8 53.1 56.2 54.7 54.8 53.1 61.4 68.9 Cm − 4 (Shore C) 56.4 55.1 54.5 53.6 51.7 54.4 53.3 53.6 51.7 59.7 67.5 Cm − 6 (Shore C) 55.9 54.7 54.2 53.3 51.5 53.7 53.1 53.3 51.5 58.7 66.8 Cc (Shore C) 55.5 54.3 53.8 52.9 51.3 53.0 52.9 52.9 51.3 57.6 66.2 Cs − Cc (Shore C) 28.9 27.5 26.9 25.9 23.8 28.1 25.3 25.9 23.8 15.1 15.6 (Cs − Cc)/(Cm − Cc) 4.9 4.3 4.7 5.1 3.8 3.9 3.7 5.1 3.8 2.1 4.2 Surface ½ × 2 × 0.5 0.4 0.3 0.3 0.2 0.7 0.2 0.3 0.2 1.0 0.7 area A (Cm − 4 − Cm − 6) Surface ½ × 2 × 1.3 1.4 1.3 1.2 1.4 1.8 1.4 1.2 1.4 1.7 1.4 area B (Cm − 2 − Cm − 4) Surface ½ × 2 × 3.7 4.2 3.7 3.2 4.5 4.0 5.1 3.2 4.5 3.4 1.0 area C (Cm − Cm − 2) Surface ½ × 2 × 5.3 4.9 4.9 4.6 4.1 5.3 4.1 4.6 4.1 1.2 1.8 area D (Cm + 2 − Cm) Surface ½ × 2 × 6.6 5.7 5.5 4.7 3.6 5.5 4.2 4.7 3.6 0.6 2.0 area E (Cm + 4 − Cm + 2) Surface ½ × 2 × 6.0 5.1 4.7 4.0 3.0 4.4 3.9 4.0 3.0 0.3 1.7 area F (Cm + 6 − Cm + 4) Surface area E + surface area F 12.6 10.8 10.2 8.7 6.6 9.9 8.1 8.7 6.6 0.9 3.7 (Surface areas: E + F) − 10.8 9.0 8.6 7.2 5.0 7.4 6.5 7.2 5.0 −1.8 1.6 (surface areas: A + B) (Surface areas: D + E) − 6.9 5.0 5.4 4.9 1.8 5.0 1.8 4.9 1.8 −3.3 1.4 (surface areas: B + C)

TABLE 4 Example Comparative Example 1 2 3 1 2 3 4 5 6 7 8 Inter- Material No. 2 No. 2 No. 2 No. 2 No. 2 No. 1 No. 1 No. 3 No. 3 No. 4 No. 5 mediate Thickness (mm) 1.51 1.51 1.50 1.51 1.52 1.51 1.51 1.49 1.52 1.32 1.46 layer Specific gravity 1.09 1.09 1.09 1.09 1.09 0.95 0.95 1.09 1.09 0.95 0.95 Material hardness (Shore C) 95 95 95 95 95 94 94 94 94 86 78 Material hardness (Shore D) 69 69 69 69 69 66 66 68 68 57 51 Inter- Diameter (mm) 41.05 41.07 41.06 41.05 41.03 41.06 41.06 41.02 41.02 40.00 40.25 mediate Weight (g) 40.5 40.6 40.6 40.5 40.4 40.8 40.8 40.5 40.4 38.7 39.4 layer- Deflection (mm) 2.84 3.06 3.22 3.52 3.72 3.31 3.56 3.58 3.80 3.95 3.24 encased Initial velocity (m/s) 77.74 77.69 77.62 77.62 77.51 77.69 77.59 77.67 77.61 77.40 77.11 sphere Surface hardness (Shore C) 98 98 98 98 98 98 97 97 97 93 92 Surface hardness (Shore D) 71 71 71 71 72 70 70 71 71 63 60 Intermediate layer surface hardness - 14 16 17 19 23 17 19 18 22 20 10 core surface hardness (Shore C) Intermediate layer surface hardness - 43 44 44 45 47 45 44 44 46 35 26 core center hardness (Shore C) Cover Material No. 8 No. 8 No. 8 No. 8 No. 8 No. 8 No. 8 No. 8 No. 8 No. 6 No. 7 Thickness (mm) 0.83 0.82 0.82 0.83 0.84 0.81 0.82 0.85 0.84 1.32 1.20 Specific gravity 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12 0.98 0.97 Material hardness (Shore C) 66 66 66 66 66 66 66 66 66 83 88 Material hardness (Shore D) 46 46 46 46 46 46 46 46 46 55 59 Ball Diameter (mm) 42.71 42.7 42.71 42.70 42.72 42.69 42.71 42.73 42.71 42.64 42.65 Weight (g) 45.3 45.3 45.3 45.3 45.3 45.6 45.6 45.4 45.3 45.3 45.3 Deflection (mm) 2.63 2.82 2.96 3.22 3.40 3.04 3.27 3.30 3.49 3.51 3.08 Initial velocity (m/s) 77.37 77.31 77.25 77.19 77.11 77.28 77.25 77.17 77.12 77.11 77.38 Surface hardness (Shore C) 85 85 85 84 85 85 84 84 85 92 95 Surface hardness (Shore D) 58 58 58 58 58 58 58 58 58 61 65 Intermediate layer surface hardness - 13 13 13 14 13 13 13 13 12 1 −3 ball surface hardness (Shore C) Deflection of core - deflection of ball (mm) 1.11 1.23 1.31 1.47 1.56 1.19 1.39 1.39 1.47 0.91 0.41 Core diameter/ball diameter 0.891 0.891 0.891 0.891 0.889 0.891 0.891 0.890 0.889 0.876 0.875 Intermediate layer thickness - 0.67 0.68 0.68 0.68 0.68 0.70 0.69 0.64 0.67 0.00 0.25 cover thickness (mm) Specific Specific gravity of cover - 0.03 0.03 0.03 0.03 0.03 0.17 0.17 0.03 0.03 0.03 0.02 gravity specific gravity of intermediate difference layer Specific gravity of intermediate −0.04 −0.04 −0.04 −0.04 −0.04 −0.22 −0.22 −0.04 −0.04 −0.25 −0.25 layer - specific gravity of core Specific gravity of core - 0.01 0.01 0.01 0.01 0.01 0.05 0.05 0.01 0.01 0.22 0.23 specific gravity of cover Initial Intermediate layer-encased 0.37 0.38 0.37 0.43 0.40 0.41 0.34 0.50 0.49 0.29 −0.27 velocity sphere - ball (m/s) difference Intermediate layer-encased 0.79 0.84 0.89 1.08 1.02 1.21 1.16 1.13 1.12 0.97 0.58 sphere - core (m/s)

The flight (W #1 and I #6), the controllability on approach shots, and the durability on repeated impact of each golf ball are evaluated by the following methods. The results are shown in Table 5.

Evaluation of Flight (W #1)

A driver (W #1) is mounted on a golf swing robot, and a spin rate and a distance traveled (total) by a ball when struck at a head speed (HS) of 45 m/s are measured. The club used is a JGR/loft angle 9.5° (2016 model) manufactured by Bridgestone Sports Co., Ltd. The rating criteria are as follows.

[Rating Criteria]

Good: Total distance is 230.0 m or more

NG: Total distance is less than 230.0 m

Evaluation of Flight (I #6)

When a number six iron (I #6) is mounted on the golf swing robot and a ball is struck at an HS of 40 m/s, a spin rate and a distance traveled (total) are measured and rated according to the following criteria. The club used is a JGR Forged I #6 (2016 model) manufactured by Bridgestone Sports Co., Ltd.

[Rating Criteria]

Good: Total distance is 164.5 m or more

NG: Total distance is less than 164.5 m

In addition, the total of the total distances struck by the two clubs (W #1 and I #6) is calculated and rated according to the following criteria.

[Rating Criteria]

Good: Total distance is 395.0 m or more

NG: Total distance is less than 395.0 m

Evaluation of Spin Rate on Approach Shots

A judgment is made based on a spin rate when a sand wedge is mounted on the golf swing robot and a ball is struck at an HS of 15 m/s. Similarly, a spin rate immediately after the ball is struck is measured by a device for measuring initial conditions. The sand wedge used is a TOURSTAGE TW-03 (loft angle 57°) 2002 model manufactured by Bridgestone Sports Co., Ltd.

[Rating Criteria]

Good: Spin rate is 4,500 rpm or more

NG: Spin rate is less than 4,500 rpm

Durability to Cracking on Repeated Impact

A durability of the golf ball is evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). The tester fires a golf ball pneumatically and causes it to repeatedly strike two metal plates installed in parallel, and the durability of the ball is an average value of the number of times of firing required until the ball cracks. In this case, the average value is a value obtained by preparing 10 balls of the same type and, by firing each ball, averaging the number of times of firing required until each of the 10 balls cracks. The tester is a horizontal COR type, and an incident velocity against the metal plates is set to 43 m/s.

[Rating Criteria]

Good: Average value is 155 times or more

NG: Average value is 154 times or less

TABLE 5 Example Comparative Example 1 2 3 1 2 3 4 5 6 7 8 Flight W#1 Spin rate 3,093 3,042 2,997 2,937 2,875 2,939 2,853 2,934 2,954 2,771 2,714 HS 45 m/s (rpm) Total (m) 234.5 232.0 230.5 225.6 224.9 232.6 230.9 228.1 225.3 223.6 231.5 Rating good good good NG NG good good NG NG NG good I#6 Spin rate 5,014 4,935 4,822 4,677 4,611 4,779 4,661 4,735 4,668 4,895 5,403 HS 40 m/s (rpm) Total (m) 164.9 165.1 165.9 168.1 166.6 167.1 167.0 166.6 167.8 163.9 162.7 Rating good good good good good good good good good NG NG W#1 + I#6 Total (m) 399.4 397.1 396.4 393.8 391.5 399.7 397.9 394.7 393.1 387.5 394.2 total Rating good good good NG NG good good NG NG NG NG Approach SW Spin rate 4,824 4,769 4,715 4,650 4,579 4,770 4,669 4,651 4,562 4,311 3,993 shots HS 15 m/s (rpm) Rating good good good good good good good good good NG NG Durability to Cracking Number 166 163 161 162 153 128 125 178 167 196 293 on Repeated Impact of cracks Rating good good good good NG NG NC good good good good

As shown in the results in Table 5, the golf balls of Comparative Examples 1 to 8 are inferior in the following respects to the golf balls according to the present invention (Examples).

In Comparative Example 1, a value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, and a deflection of the ball is larger than 3.00 mm. As a result, a distance on shots with a driver (W #1) is inferior.

In Comparative Example 2, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, and the deflection of the ball is larger than 3.00 mm. As a result, the distance on shots with a driver (W #1) is inferior, and a durability to cracking on repeated impact is poor.

In Comparative Example 3, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, the deflection of the ball is larger than 3.00 mm, and a specific gravity of the intermediate layer material is smaller than 1.05. As a result, the durability to cracking on repeated impact is poor.

In Comparative Example 4, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, the deflection of the ball is larger than 3.00 mm, and the specific gravity of the intermediate layer is smaller than 1.05. As a result, the durability to cracking on repeated impact is poor.

In Comparative Example 5, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, and the deflection of the ball is larger than 3.00 mm. As a result, a distance on shots with a driver (W #1) is inferior.

In Comparative Example 6, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is larger than 1.00 m/s, and the deflection of the ball is larger than 3.00 mm. As a result, a distance on shots with a driver (W #1) is inferior.

In Comparative Example 7, the deflection of the ball is larger than 3.00 mm, and the specific gravity of the intermediate layer is smaller than 1.05. As a result, a distance on shots with both the driver (W #1) and an iron (I #6) is inferior, and a spin rate on approach shots becomes lower.

In Comparative Example 8, the value of (initial velocity of intermediate layer-encased sphere−initial velocity of core) is smaller than 0.60 m/s, the initial velocity of the intermediate layer-encased sphere is smaller than the initial velocity of the ball, the deflection of the ball is larger than 3.00 mm, and the specific gravity of the intermediate layer is smaller than 1.05. As a result, the distance on shots with the iron (I #6) is inferior, and the spin rate on approach shots becomes lower.

Japanese Patent Application No. 2022-123651 is incorporated herein by reference. Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an intermediate layer, and a cover, the core being formed of a rubber composition into a single layer or a plurality of layers, the intermediate layer and the cover being both formed of a single-layer resin composition, wherein a relationship between an initial velocity of the core, an initial velocity of a sphere (intermediate layer-encased sphere) obtained by encasing the core with the intermediate layer, and an initial velocity of a sphere (ball) obtained by encasing the intermediate layer-encased sphere with the cover satisfies the following two conditions: (initial velocity of ball)<(initial velocity of intermediate layer-encased sphere) 0.60≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.00 (m/s) and a deflection when the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is 3.00 mm or less, and a specific gravity of the intermediate layer is 1.05 or more.
 2. The multi-piece solid golf ball of claim 1, wherein assuming that when each sphere of the core and the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and deflections (mm) are denoted by C (mm) and B (mm) respectively, a value of C-B is 1.45 mm or less.
 3. The multi-piece solid golf ball of claim 1, wherein the following condition is satisfied: ball surface hardness<surface hardness of intermediate layer-encased sphere>core surface hardness (where the surface hardness of each sphere means Shore C hardness).
 4. The multi-piece solid golf ball of claim 1, wherein the resin composition of the intermediate layer contains a high-acid ionomer resin having an acid content of 16 wt % or more.
 5. The multi-piece solid golf ball of claim 1, wherein the intermediate layer contains an inorganic particulate filler.
 6. The multi-piece solid golf ball of claim 1, wherein a difference between a specific gravity of the cover and the specific gravity of the intermediate layer is 0.15 or less.
 7. The multi-piece solid golf ball of claim 1, wherein the following condition is satisfied: cover thickness<intermediate layer thickness.
 8. The multi-piece solid golf ball of claim 1, wherein the core has a diameter of 36.7 to 40.1 mm and has a hardness profile in which, letting Cc be a Shore C hardness at the center of the core, Cm be a Shore C hardness at a midpoint M between the center and the surface of the core, Cm−2, Cm−4, and Cm−6 be respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M, Cm+2, Cm+4, and Cm+6 be respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the center M, and Cs be a Shore C hardness at the surface of the core, the following surface areas A to F are defined: surface area A: ½×2×(Cm−4−Cm−6) surface area B: ½×2×(Cm−2−Cm−4) surface area C: ½×2×(Cm−Cm−2) surface area D: ½×2×(Cm+2−Cm) surface area E: ½×2×(Cm+4−Cm+2) surface area F: ½×2×(Cm+6−Cm+4) and the following condition is satisfied: (surface area E+surface area F)−(surface area A+surface area B)≥2.0.
 9. The multi-piece solid golf ball of claim 8, wherein the following condition is satisfied: (surface area D+surface area E)−(surface area B+surface area C)≥2.0. 